1. Field of the Invention
The present invention relates to planar magnetron sputtering, and more specifically, to a magnet track and a sweeping method for planar magnetron sputtering.
3. Description of Related Art
Sputtering describes a number of physical techniques commonly used in, for example, the semiconductor industry for the deposition of thin films of aluminum and aluminum alloys, refractory metal silicides, gold, copper, titanium-tungsten, tungsten, molybdenum, and less commonly, silicon dioxide and silicon on an item, for example, a wafer during processing. In general, the techniques involve producing a gas plasma of ionized inert gas "particles" (atoms or molecules) by using an electrical field in an evacuated chamber. The ionized particles are then directed toward an electrically-biased target and collide with it. As a result of the collisions, free atoms of the target material are released from the surface of the target, essentially converting the target material to its gas phase. A portion of the free atoms which escape the target surface condense and form (deposit) a thin film on the surface of the wafer, which is located a short distance in front of the target.
One common sputtering technique is magnetron sputtering. Magnetron sputtering uses a magnetic field to concentrate the sputtering action so that it occurs at a higher rate and a lower process pressure. The magnets are normally positioned behind the target, with their north-south axes parallel to the surface of the target. Alternatively, magnet pairs may be oriented with their axes perpendicular to the surface of the target, the magnets in each pair having their north and south poles, respectively, directed towards the target. With either orientation, the lines of the magnetic field penetrate the target and form arcs over its surface. The magnetic field helps to capture free electrons in an area near the surface of the target. The resulting increased concentration of free electrons produces a greater density of inert gas ions and enhances the efficiency of the sputtering process.
If the magnet generating the magnetic field is stationary, then continuous sputtering will generate a hot spot and consume the sputtering target thickness at that location quickly. To avoid contaminating the wafer, the sputtering must be stopped before the wear pattern has consumed the full thickness of the target material at any point. If any point on the target plate behind the target is reached, sputtering of the target backplate material (usually copper) will occur, contaminating the vacuum chamber and the wafer with copper. Because the pattern of target utilization is normally not uniform, in practice sputtering must be stopped when a significant percentage of the target still remains.
In designing the target and its associated magnetic field, two main objectives are a uniform erosion of the target and uniform deposition of target material on the wafer. To increase the uniformity of target erosion and thereby improve target utilization, magnets in a magnet housing have been moved in various oscillatory patterns. These techniques may provide greater target utilization than stationary magnets, but they also produce grooves, racetracks or other non-uniform wear patterns as the target is consumed. The non-uniform target utilization takes place because the magnets and their associated magnetic field, as they are configured and moved, do not dwell uniformly over the target. Ideally, to produce uniform target erosion, the magnetic field should remain for an equal amount time over each unit (e.g., square cm) of the target's area. This is referred to as the "dwell time" of the magnetic flux. When the target is circular, which is usually the case, and the magnet is rotated about the center of the target, the equalization of dwell times requires that the magnetic field be present for proportionally greater periods of time near the periphery of the target than near its center. This is because the amount of area enclosed within a given annular segment of the target increases with increasing radius.
Another important design objective is to insure that some degree of sputtering occurs in all areas of the target. Otherwise, target material which is "back scattered" and redeposited on the target surface in areas where sputtering does not occur may form relatively loose accumulations (flakes) which can become detached from the target and contaminate the wafer.
One approach using a fixed-magnet sputtering apparatus is described in U.S. Pat. No. 4,680,061, issued Jul. 14, 1987, to Lamont, and U.S. Pat. No. 4,100,055, issued Jul. 11, 1978 to Rainey. The target is shaped like a ring, which is said to provide good deposition uniformity without using relative movement between the source and the wafer, as had previously been done.
Another approach involving a magnetic field source in a planar magnetron sputtering apparatus is described in U.S. Pat. No. 4,444,643, issued Apr. 24, 1984, to Garrett. In contrast to fixed magnetron sputtering apparatus, the Garrett apparatus moves a magnetic field source across the non-vacuum side of the target to sweep magnetic flux lines over the target surface. Since maximum erosion of a target occurs where lines of magnetic flux are parallel with the surface of the target, the sweeping is said to avoid the "racetrack" grooves found in prior sputtering devices having fixed magnetic field sources, and thereby provides greater uniformity of target erosion. The magnetic field source of Garrett's apparatus includes a magnet and ring subassembly, the magnetically permeable ring thereof having a plurality of permanent magnets arranged radially inward from the inner circumference of the ring and in paired symmetry. The axial arrangement of the magnets is said to cause the creation of a series of loop pairs of flux in planes normal to the plane of the target and long diameters through the paired magnets.
Another swept field approach is described in U.S. Pat. No. 4,714,536, issued Dec. 22, 1987 to Freeman et al. The magnet assembly of Freeman et al. is rotated about a central axis relative to the target surface and simultaneously rotated about a second axis spaced from the central axis, with the magnet assembly being mounted off-center with respect to the second axis. The resulting pattern is said to be essentially an epicycloid, which is displaced throughout the axis of rotation with each successive revolution. The particular path traced by the magnet assembly is dependent upon the radii and gear ratios of the driving motor assembly. The magnet assembly itself contains permanent magnets mounted with their north-south axes aligned with radii of the cup-shaped holder, such that the north pole of each of the magnets is adjacent to the center of the holder.
Our copending application 07/632,444, filed Dec. 21, 1990, abandoned, describes an arrangement in which a magnet track in the form of a closed curve having at least one cusp and at least one loop is rotated behind the surface of the target while varying the instantaneous center of rotation. This arrangement has been formed to provide a high degree of uniformity of erosion in an annular region of the target.
Conventional cooling methods in prior art sputtering devices employ a cavity behind or internal to a plate being heated by sputtering; see, e.g., the patents to Lamont, Rainey, Garrett and Freeman et al. mentioned above, as well as U.S. Pat. No. 3,956,093 issued May 11, 1976 to McLeod; U.S. Pat. No. 4,116,806 issued Sep. 26, 1978 to Love et al.; and U.S. Pat. No. 4,175,030 issued Nov. 20, 1979 to Love et al. In these patents, water or another liquid coolant is provided through an opening to internal passages of the device or a cavity in the device. A separate opening allows the coolant to be discharged from the device. The devices having internal passages route the coolant through these passages, which are adjacent to the heat generating elements of the device, to cool them. In devices where the coolant is routed to a cavity, the direction of the flow of coolant as well as the movement of a mechanism inside the cavity contributes to the agitation of the coolant as it cools the heat generating elements before the coolant exits the cavity. These prior art methods for cooling devices having internal cavities sometimes create large and often random differences in actual coolant flow and in the temperature of the target plate from one side to the other. The coolant chamber is pressurized as coolant flows. Pressurizing the cavity in contact with the target plate requires that it be strengthened, because the pressure within the coolant chamber when added to the vacuum pressure in the evacuated sputtering cavity provides a large differential pressure across the target plate. The target plate must be sized to support this large differential pressure.
Cooling has also been accomplished by drilling gun bore-type channels in the target backing plate similar to the fixed cooling passages in the prior art. The location of these channels causes localized hot spots between the channels, however, and temperature increases away from the location of coolant flow. Also, drilling, locating and connecting piping to these channels unnecessarily complicates the construction of the target backing plate.
The distance between the magnets and the surface of the target affects the degree to which sputtering is concentrated by the magnet's magnetic field. For a given magnet design, a short distance causes sputtering to be more highly concentrated than a greater distance. The highest sputtering concentration occurs if a target having a negligible thickness is mounted directly on the magnets. Every additional increment of distance between the surface of the target and the magnet reduces the influence of the magnetic field upon sputtering of the target. A thick target backing plate is necessary in a pressurized coolant chamber device to obtain the additional strength needed to support the pressure behind the target backing plate, and it is particularly necessary in a device using a target backing plate which has been drilled for cooling and therefore requires additional thickness for strength. The use of a thick target backing plate adds extra thickness to the target plate assembly. To avoid a reduction in magnetic flux at the target's surface, a thin target must be substituted to maintain the same distance between the magnets and the target's surface. Alternatively, a reduced magnetic flux at the target surface, as a result of the increased distance between the magnets and the surface of the target, must be accepted. Therefore, the need to increase the thickness of the target backing plate to provide more strength is in conflict with the desire to reduce the distance between the surface of the target and the magnets to concentrate the location of sputtering.
Despite considerable improvement in the engineering of targets and magnetic field sources for planar magnetron sputtering equipment, uniform erosion of the target and uniform deposition of pure target material on the wafer have not been totally achieved. There is a particular need to provide a highly uniform deposition where the sputtered material is to be deposited on a large diameter wafer.